Current, flexible, thick film EL lamps are typically formed on a release layer, such described in U.S. Pat. N0. 5,856,030 (Burrows). A first insulating layer overlies the release layer and is covered by a first electrode. A phosphor layer overlies the first electrode and a dielectric layer overlies the phosphor layer. A second electrode is deposited over the dielectric layer. A second insulator overlies the second electrode. The electrodes are a conductive polymer, such as PEDOT/PSS (Poly-3,4-ethylenedioxythiophene/ polystyrenesulfonic acid) sold under the tradename “Orgacon™” by Agfa-Gevaert N.V. The conductive polymer can be translucent or opaque, depending upon composition or additives.
Bus bars, located at least along one edge of the lit area, are used to improve the conductivity of the electrodes. Typically, the bus bars are screen printed from an ink containing resin and conductive particles, such as carbon or silver. Silver particles are desirable because they provide good conductivity and reflect light.
Silver particles contribute to a problem known as “silver migration”, a dendritic growth of silver oxide crystals into the lit area of the lamp, causing black spots and electrical short circuits. The problem has long been known in the art and has been associated with DC bias on a lamp; e.g. see U.S. Pat. No. 4,143,297 (Fischer). The Fischer patent proposes a symmetrical structure to avoid “one-sided space charge conditions”, i.e., DC bias. More typically, barrier layers have been proposed to minimize silver migration. For example, see U.S. Pat. No. 5,697,322 (Hay et al.), U.S. Pat. No. 5,811,930 (Krafcik et al.), U.S. Pat. No. 6,586,873 (Mizutani et al.), and U.S. Pat. No. 6,965,196 (Murasko et al.).
In portable electronic devices, automotive displays, and other applications where the power source is a low voltage battery, an EL lamp is powered by a driver that converts low voltage direct current into high voltage alternating current. In order for an EL lamp to glow sufficiently, a peak-to-peak voltage in excess of about one hundred and twenty volts is necessary. The actual voltage depends on the construction of the lamp and, in particular, the field strength within the phosphor powder.
U.S. Pat. No. 4,210,848 (Suzuki et al.) discloses a driver having a switching bridge output. The bridge acts as a double pole double throw switch to alternate current through the EL lamp at low frequency. U.S. Pat. No. 5,313,141 (Kimball) discloses a driver that produces AC voltage without a bridge. A plurality of drivers are commercially available in integrated circuit form using either technology.
Unless components are carefully matched, and they are not because it is too expensive, there is usually a slight asymmetry in the waveform applied to an EL lamp. The slight asymmetry causes a DC bias to accumulate over time and several techniques have been proposed for preventing the accumulation of charge or for more thoroughly discharging a lamp, thereby reducing DC bias. Unless the DC bias is removed completely, or silver is not used, the problem of silver migration exists in the prior art. There are also test results suggesting that moisture and field intensity contribute to silver migration.
In view of the foregoing, it is therefore an object of the invention to provide an apparatus and a method for eliminating silver migration in EL lamps without eliminating silver.
A further object of the invention is to eliminate silver migration regardless of moisture or field intensity.
Another object of the invention is to simplify the construction of EL lamps, and reduce costs, by eliminating barrier layers for reducing silver migration.